Great points to make. I just get so frustrated when I keep seeing "well no critique here, but pretty horse". Yes, it is wonderful that you think the horse is pretty, however quite frankly, I could care less if you think that the horse is ugliest thing in the world, that's not what people are looking for. People place their horses on the critique section to be evaluated for number of reasons, not just if it is pretty. It's fine that people want to do this, but perhaps go to the horse picture section of site.
As far as the younger posters goes. Yes, you are correct in that a "lighter" critique would be appropriate, and a complement sandwich is a wonderful thing to point out. However, not everyone is going to take that into consideration. Like you stated, horsepeople can be way too opinionated, I know that I am guilty of this myself. So I think it stands to all ages (young and old) please remember that when you place your horse on that board, you may not like all the feedback that you receive. However, you are the one putting yourself out there. If it really bothers you, then skip reading over what you don't like and move on.